Procedures for making perbrominated polyphenylene ethers are known. For example, decabromodiphenyl ether (referred to as "Decabrom") is a commercial fire retardant. It can be made by adding molten diphenyl ether to a large stoichiometric excess of liquid bromine containing a halogenation catalyst such as aluminum halide (U.S. Pat. NO. 3,965,197). Although it is not a solvent, the large excess of bromine acts as a liquid medium in which the product is suspended and from which it can be readily recovered.
In some uses it is preferred to have only a partially brominated flame retardant because it might have more desirable physical properties in a particular substrate. For example a partially brominated diphenyl ether is sold as octabromodiphenyl ether (referred to as "Octabrom") although its actual composition is a mixture of partially brominated diphenyl ether. The commercial grade product contains about 6-9 bromine atoms per molecule.
Using the aluminum halide bromination catalyst as practiced in U.S. Pat. No. 3,965,197, the degree of bromination can only be limited by restricting the amount of bromine at about the stoichiometric amount required to insert eight bromine atoms per molecule. Also the product is very colored (yellow to brown) requiring extensive purification if a light colored product is required.